For safety, pneumatic tires have been historically sought which have a means of retarding or preventing their deflation upon being punctured. Many methods and tire constructions have been suggested and used for this purpose, mostly without significant commercial success for conventional passenger vehicles such as automobiles which are to be driven over typical roadways; this lack of commercial success has been due to high cost, complexity as to design and poor sealant performance. Fluid puncture sealants which seal by flowing into the puncture hole have not been entirely successful because they tend to cause the tire to become out of balance or tend to have varying flow properties over a wide temperature range. Central cores of cellular material which will physically maintain the tire shape when punctured have been suggested. However, such cores place a serious restriction on the vehicular maximum speed or endurance due to degradation of the core.
Puncture sealing tubeless tires have previously been proposed, containing, in the area of the tire normally most subject to punctures (that is, the undertread or the area extending across the crown of the tire at least from one shoulder to the other), a layer of sealant composition which has viscoelastic and adhesive qualities such that the composition tends to stick to a puncturing object, and, when the puncturing object is withdrawn, tends to flow into the opening or puncture, forming a plug which seals the opening against loss of air from the tires. Unfortunately, it has proven difficult to provide a composition which would flow into the puncture hole and yet have sufficient viscosity to prevent it from flowing at elevated temperatures such as exist in an automobile pneumatic tire under operating conditions. The problem is complicated by the extreme centrifugal force to which the composition is subjected as the tire rotates at high speed, since such centrifugal force tends to cause the composition to flow into the central crown area, leaving the areas near the shoulder unprotected. Furthermore, it has proven difficult to provide a sealant composition which would retain this desired balance of viscosity, adhesion and conformability over an extended period of service.
Various elastomer-based compositions, both cured and uncured, have been proposed as puncture sealants. In the uncured state, although they may function as sealants, they will sometimes tend to "cold flow" or flow at elevated temperatures such as are encountered in tires during use. This flow is undesirable. When they are crosslinked (cured) to prevent flow, these materials can lose the adhesion and conformability of the uncured state, and thus no longer act as sealants.